Single-crystal 4H-SiC substrate

ABSTRACT

A single-crystal 4H-SiC substrate includes a 4H-SiC bulk single-crystal substrate; and an epitaxial first single-crystal 4H-SiC layer on the 4H-SiC bulk single-crystal substrate and having recesses. The recesses have a diameter no smaller than 2 μm and no larger than 20 μm. The recesses have a depth no smaller than 0.01 μm and no larger than 0.1 μm. A single-crystal 4H-SiC substrate also includes a 4H-SiC bulk single-crystal substrate; and an epitaxial first single-crystal 4H-SiC layer on the 4H-SiC bulk single-crystal substrate and having recesses. The density of the recesses in the epitaxial first single-crystal 4H-SiC layer is at least 10/cm 2 , and the epitaxial first single-crystal 4H-SiC layer has a defect density no larger than 2/cm 2 .

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a single-crystal 4H-SiC substrate andmethod for manufacturing the same having a reduced crystal defectdensity.

Background Art

Recently, much attention has been given to the use of silicon carbide(or SiC) as a material primarily for power control devices, since SiC issuperior to silicon in terms of breakdown field strength, saturateddrift velocity, and thermal conductivity. Power devices formed of SiCcan be configured to have substantially reduced power loss and a reducedsize, allowing power saving in power conversion. Therefore, these powerdevices can be used to enhance the performance of electric vehicles andthe functionality of solar cell systems, etc., and hence are a keyelement in creating a low-carbon society.

The doping density and the thickness of a substrate on which an SiCpower device is to be formed are determined substantially by thespecifications of the device and therefore are typically required to becontrolled more accurately than the doping density and the thickness ofa bulk single-crystal substrate. In order to meet this requirement, anactive region for the semiconductor device is epitaxially grown on a4H-SiC bulk single-crystal substrate beforehand by thermal chemicalvapor deposition (thermal CVD), etc. It should be noted that the term“active region,” as used herein, refers to a region having an accuratelycontrolled thickness and an accurately controlled doping density in itscrystal structure.

4H-SiC bulk single-crystal substrates inherently contain screwdislocations, which propagate in the c-axis direction, edgedislocations, and basal plane dislocations, which propagateperpendicular to the c-axis. These dislocations propagate into theepitaxial film grown on the substrate. Furthermore, new dislocationloops and stacking faults are introduced into the substrate during theepitaxial growth process. These crystal defects may degrade thebreakdown voltage characteristics, reliability, and yield of the deviceformed using the SiC substrate and thereby prevent the practical use ofthe device.

It should be noted that methods of manufacturing a single-crystal 3C-SiCsubstrate have been proposed in which a single-crystal 3C-SiC layer isformed to have a flat surface interspersed with surface pits, whichserves to reduce crystal defects (see, e.g., Japanese Laid-Open PatentPublication No. 2011-225421).

SUMMARY OF THE INVENTION

Since 3C-SiC is a cubic crystal and 4H-SiC is a hexagonal crystal, theyhave different crystal structures, i.e., different atomic arrangements,and hence have substantially different optimum growth conditions. Forexample, whereas 3C-SiC has an optimum growth temperature range of1000-1100° C., 4H-SiC has a very high optimum growth temperature rangeof 1600-1800° C. As a result, the currently available methods forreducing crystal defects in a single-crystal 3C-SiC substrate cannot beapplied to single-crystal 4H-SiC substrates. Heretofore there has beenno known method for reducing crystal defects in a single-crystal 4H-SiCsubstrate.

In view of the above-described problems, an object of the presentinvention is to provide a single-crystal 4H-SiC substrate and method formanufacturing the same having a reduced crystal defect density.

According to the present invention, a method for manufacturing asingle-crystal 4H-SiC substrate includes: preparing a 4H-SiC bulksingle-crystal substrate having flatness; and forming a firstsingle-crystal 4H-SiC layer having recesses on the 4H-SiC bulksingle-crystal substrate, using an epitaxial method, wherein thicknessof the first single-crystal 4H-SiC layer is X measured in micrometers(μm), the recesses have a diameter Y, measured in micrometers, nosmaller than 0.2*X and no larger than 2*X, and the recesses have a depthZ, if measured in micrometers (μm), no smaller than (0.95*X+0.5)10⁻³ andno larger than 10*X*10⁻³.

The present invention makes it possible to reduce a crystal defectdensity of a single-crystal 4H-SiC substrate.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are cross-sectional views showing the method ofmanufacturing a single-crystal 4H-SiC substrate in accordance with thefirst embodiment.

FIG. 3 is an optical micrograph image of recesses formed in the growthsurface of the single-crystal 4H-SiC layer 3.

FIG. 4 is a diagram showing the diameter of the recesses as a functionof the thickness of the epitaxial film

FIG. 5 is a diagram showing the depth of the recesses as a function ofthe thickness of the epitaxial film.

FIG. 6 is a diagram showing the defect density of the single crystal4H-SiC layer as a function of the recess density of the surface of thelayer.

FIG. 7 is a cross-sectional view showing the method of manufacturing asingle-crystal 4H-SiC substrate in accordance with the secondembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A single-crystal 4H-SiC substrate and method for manufacturing the sameaccording to the embodiments of the present invention will be describedwith reference to the drawings. The same components will be denoted bythe same symbols, and the repeated description thereof may be omitted.

First Embodiment

A method of manufacturing a single-crystal 4H-SiC substrate inaccordance with a first embodiment of the present invention will bedescribed. FIGS. 1 and 2 are cross-sectional views showing the method ofmanufacturing a single-crystal 4H-SiC substrate in accordance with thefirst embodiment.

The method begins by preparing a 4H-SiC bulk single-crystal substrate 1misoriented by 4 degrees from the (0001) plane (or C-plane) toward a<11-20> direction, as shown in FIG. 1. (The completed single-crystal4H-SiC substrate of the present embodiment will have a principal surfacein the (0001) plane.) It should be noted that the misorientation angleneed not necessarily be 4 degrees, but may be in the range of 2-10degrees.

Specifically, the 4H-SiC bulk single-crystal substrate 1 is planarizedby mechanical polishing and chemical mechanical polishing using anacidic or alkaline solution. Further, the substrate 1 is ultrasonicallycleaned using acetone to remove organic matter. The 4H-SiC bulksingle-crystal substrate 1 is then subjected to the so-called RCAcleaning Specifically, the 4H-SiC bulk single-crystal substrate 1 isimmersed in a mixture (1:9) of aqueous ammonia and hydrogen peroxidesolution for 10 minutes after the mixture has been heated to 75° C. (±5°C.). The substrate 1 is then immersed in a mixture (1:9) of hydrochloricacid and hydrogen peroxide solution after the mixture has been heated to75° C. (±5° C.). Further, the 4H-SiC bulk single-crystal substrate 1 isimmersed in an aqueous solution containing approximately 5 volumepercent of hydrofluoric acid, and then is subjected to substitutiontreatment using purified water, thus cleaning the surface of the 4H-SiCbulk single-crystal substrate 1.

Next, the 4H-SiC bulk single-crystal substrate 1 is introduced into aCVD apparatus, which is then evacuated to approximately 1×10⁻⁷ kPa. Thesubstrate 1 is then heated to approximately 1400-1700° C. and annealedin a reducing gas atmosphere. Then, as shown in FIG. 2, material gasesare supplied to the growth furnace so that a single-crystal 4H-SiC layer3 is epitaxially grown on the 4H-SiC bulk single-crystal substrate 1 andhas recesses 2 having a diameter of 2-20 μm and a maximum depth of10-100 nm. These material gases include, e.g., silane gas (SiH₄) whichis used as a Si atom source, propane gas (C₃H₈) which is used as a Catom source, and nitrogen gas serving as an N-type dopant. In thisexample, the single-crystal 4H-SiC layer 3 is formed to a thickness of10 μm by supplying SiH₄ gas at a flow rate of 500 sccm and C₃H₈ gas at aflow rate of 200 sccm. Further, nitrogen gas serving as an N-type dopantis supplied so that the substrate interface has a carrier concentrationof 1×10¹⁷/cm³ and the active region has a carrier concentration of8×10¹⁵/cm³. The supply of material gas is then stopped and thetemperature of the substrate 1 is decreased to room temperature.

The present inventor has found that minute recesses 2 are formed in thegrowth surface of the single-crystal 4H-SiC layer 3 if the temperatureand the pressure in the growth furnace are appropriately set during thegrowth of the layer 3. FIG. 3 is an optical micrograph image of recessesformed in the growth surface of the single-crystal 4H-SiC layer 3. Thedensity of the recesses 2 was found, using an optical microscope, to beapproximately 600/cm². The shape of the recesses 2 was observed under anatomic force microscope and found to be a nonsymmetrical elliptical conehaving a diameter of 2-20 micrometer (μm) and a maximum depth of 10-100nanometer (nm). More detailed experiments were conducted repeatedly andrevealed that the size of the recesses varies with the thickness of theformed epitaxial film and that the greater the thickness of the film,the greater the diameter and the depth of the recesses. FIG. 4 is adiagram showing the diameter of the recesses, measured in micrometers(μm), as a function of the thickness of the epitaxial film, measured in(μm). FIG. 5 is a diagram showing the depth of the recesses measured innanometers (nm), as a function of the thickness of the epitaxial film,measured in micrometers (μm). The results of the experiments show that,when the thickness of the epitaxial film is X (μm), the diameter Y (μm)of the recesses is no less than 0.2*X (μm) and no more than 2*X (μm),and the depth Z of the recesses, when measured in micrometers (μm), isno smaller than (0.95*X+0.5 10⁻³, and no larger than 10*X*10⁻³. FIG. 5shows that when the thickness of the film is 8 micrometers (μm), themaximum recess depth within the scope of the invention is 80 nanometers(nm), i.e., 0.08 micrometers (μm) and the minimum recess depth withinthe scope the invention is 8.1 nanometers (nm), i.e., 0.0081 micrometers(μm).

FIG. 6 is a diagram showing the defect density of the single crystal4H-SiC layer as a function of the recess density of the surface of thelayer. The defect density was obtained by photoluminescence topography(PL-TOPO). It should be noted that the term “defect,” as used herein,corresponds to or is defined as an anomalously luminous region observedby PL-TOPO. In the case of a conventional substrate having asingle-crystal 4H-SiC layer formed under typical conventional growthconditions, the density of the recesses in the surface of thesingle-crystal 4H-SiC layer was less than 10/cm²; substantially norecesses were found under an optical microscope. In this case, thedefect density was 60/cm² or more. It should be noted that theelectrodes of some devices have a large surface area (e.g., 1-2 mmsquare), in which case if these devices are formed on a conventionalsingle-crystal 4H-SiC substrate, then more than one defect will belocated under the electrodes, resulting in degraded breakdown voltagecharacteristics of the device.

In the case of the single-crystal 4H-SiC substrate of the presentembodiment, on the other hand, the defect density is considerably low(namely, 2/cm²), as compared with prior art substrates of the same type,since the density of the recesses 2 formed in the surface of thesingle-crystal 4H-SiC layer 3 is 10/cm² or more. When the density of therecesses 2 was 1500/cm², the defect density was found to be extremelylow (namely, 1/cm²).

As described above, in the present embodiment, the single-crystal 4H-SiClayer 3 is grown to have recesses having a diameter Y (μm) of no lessthan 0.2*X (μm) and no more than 2*X (μm), and a depth Z (nm) of no lessthan (0.95*X (μm)+0.5 (nm)) and no more than 10*X (μm), where X is thethickness of the single-crystal 4H-SiC layer 3 in μm. This results in areduced crystal defect density of the single-crystal 4H-SiC substrate.Further, the use of this high quality single-crystal 4H-SiC substratemakes it possible to improve the breakdown voltage characteristics,reliability, and yield of the device formed thereon.

It should be noted that an organic metal material containing Al, B, orBe and serving as a P-type dopant may be supplied as necessary duringthe formation of the single-crystal 4H-SiC layer 3. Further, achlorine-containing gas may be additionally supplied to increase thegrowth rate of the layer. It has been found that the growth rate of thesingle-crystal 4H-SiC layer 3 can be varied by varying the flow rate ofmaterial gas and that the above advantages of the present embodiment canbe achieved regardless of whether the growth rate is 1 μm/h or 10 μm/h.

It has also been found that the density of the recesses 2 can beadjusted by appropriately setting the temperature and the pressure inthe growth furnace. It should be noted, however, that such setting orconditions may be greatly dependent on the structure and internalconfiguration of the furnace of the CVD apparatus; that is, theappropriate conditions vary with the CVD apparatus used.

Second Embodiment

A method of manufacturing a single-crystal 4H-SiC substrate inaccordance with a second embodiment of the present invention will bedescribed. FIG. 7 is a cross-sectional view showing the method ofmanufacturing a single-crystal 4H-SiC substrate in accordance with thesecond embodiment.

First, as in the first embodiment, a single-crystal 4H-SiC layer 3having a thickness of 300 nm is epitaxially grown so that that itsgrowth surface has recesses 2. It should be noted that the thickness ofthe single-crystal 4H-SiC layer 3 need not necessarily be 300 nm, butmay be in the range of from 50 nm to 10 μm. Then, as shown in FIG. 7, asingle-crystal 4H-SiC layer 4 having a thickness of 10 μm is epitaxiallygrown on the single-crystal 4H-SiC layer 3 so as to bury the recesses 2.

Specifically, the single-crystal 4H-SiC layer 4 is formed by supplyingSiH₄ gas at a flow rate of 900 sccm and C₃H₈ gas at a flow rate of 360sccm and further supplying nitrogen gas serving as an N-type dopant sothat the layer 4 has a carrier concentration 8×10¹⁵/cm³. The supply ofmaterial gas is then stopped and the temperature of the substrate isdecreased to room temperature. Except for this feature, thesingle-crystal 4H-SiC substrate of the second embodiment is similar inconfiguration and manufacture to the single-crystal 4H-SiC substrate ofthe first embodiment.

It should be noted that the recesses 2 of the single-crystal 4H-SiClayer 3 can be buried under the single-crystal 4H-SiC layer 4 byappropriately setting the growth temperature and other growth conditionsso as to grow the single-crystal 4H-SiC layer 4 primarily in a step flowgrowth mode.

The density of the recesses in the surface of the single-crystal 4H-SiCsubstrate of the present embodiment was found, using an opticalmicroscope, to be very low (namely, approximately 1/cm²). Further, a 10μm by 10 μm square area of the surface was observed under an atomicforce microscope and found to have an average roughness (Ra) of 0.3 nmor less without anomalous growth called step bunching, indicating thatthe surface was in very good conditions. Further, the defect density ofthe single-crystal 4H-SiC substrate was found, using PL-TOPO, to be verylow (namely, 2/cm²). The low defect density of the completedsingle-crystal 4H-SiC substrate resulted from the formation of thesingle-crystal 4H-SiC layer 3 on the 4H-SiC bulk single-crystalsubstrate 1, which served to reduce the defect density.

In the present embodiment, the single-crystal 4H-SiC layer 4 is formedto bury the recesses 2 of the single-crystal 4H-SiC layer 3. Thisreduces crystal defects and improves the flatness of the surface of thesingle-crystal 4H-SiC substrate.

Thus, the single-crystal 4H-SiC substrate of the second embodiment hastwo single-crystal 4H-SiC layers, one on top of the other, and the lowersingle-crystal 4H-SiC layer has recesses in its surface. In suchsingle-crystal 4H-SiC substrates, too, the density of the recesses andthe defect density are related to each other in the manner shown in FIG.6 described in connection with the single-crystal 4H-SiC substrate ofthe first embodiment, which has only one single-crystal 4H-SiC layer.Further, in the single-crystal 4H-SiC substrate of the secondembodiment, the single-crystal 4H-SiC layer 3 need not necessarily beformed in contact with the 4H-SiC bulk single-crystal substrate 1; forexample, the single-crystal 4H-SiC layer 3 may be sandwiched in thesingle-crystal 4H-SiC layer 4. This also serves to reduce crystaldefects. Thus, the position of the single-crystal 4H-SiC layer 3relative to the surface of the 4H-SiC bulk single-crystal substrate 1may be selected according to the specifications of the device formed. Asa result, it is possible to control the defect density of thesingle-crystal 4H-SiC substrate of the second embodiment whileaccurately controlling the carrier concentration and thickness of theactive region.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

The entire disclosure of Japanese Patent Application No. 2013-064365,filed on Mar. 26, 2013 including specification, claims, drawings andsummary, on which the Convention priority of the present application isbased, is incorporated herein by reference in its entirety.

What is claimed is:
 1. A single-crystal 4H-SiC substrate comprising: a 4H-SiC bulk single-crystal substrate; and an epitaxial first single-crystal 4H-SiC layer on the 4H-SiC bulk single-crystal substrate and having recesses, wherein the recesses have respective inner surfaces that consist of smooth curved surfaces, the recesses have a diameter no smaller than 2 μm and no larger than 20 μm, and the recesses have a depth no smaller than 0.01 μm and no larger than 0.1 μm.
 2. The single-crystal 4H-SiC substrate according to claim 1, wherein the epitaxial first single-crystal 4H-SiC layer has a thickness no smaller than 0.3 μm and no larger than 10 μm.
 3. The single-crystal 4H-SiC substrate according to claim 1, wherein density of the recesses in the epitaxial first single-crystal 4H-SiC layer is at least 10/cm², and the epitaxial first single-crystal 4H-SiC layer has a defect density no larger than 2/cm².
 4. The single-crystal 4H-SiC substrate according to claim 1, comprising an epitaxial second single-crystal 4H-SiC layer on the epitaxial first 4H-SiC single-crystal layer, burying and completely filling the recesses, without any gaps between the smooth curved surfaces of the recesses and the epitaxial second single-crystal 4H-SiC layer.
 5. A single-crystal 4H-SiC substrate comprising: a 4H-SiC bulk single-crystal substrate; and an epitaxial first single-crystal 4H-SiC layer on the 4H-SiC bulk single-crystal substrate and having recesses, wherein the recesses have respective inner surfaces that consist of smooth curved surfaces, density of the recesses in the epitaxial first single-crystal 4H-SiC layer is at least 10/cm², and the epitaxial first single-crystal 4H-SiC layer has a defect density no larger than 2/cm².
 6. The single-crystal 4H-SiC substrate according to claim 5, further comprising an epitaxial second single-crystal 4H-SiC layer on the epitaxial first single-crystal 4H-SiC layer, burying and completely filling the recesses, without any gaps between the smooth curved surfaces of the recesses and the epitaxial second single-crystal 4H-SiC layer.
 7. The single-crystal 4H-SiC substrate according to claim 5, wherein the epitaxial first single-crystal 4H-SiC layer has a thickness no smaller than 0.3 μm and no larger than 10 μm.
 8. A single-crystal 4H-SiC substrate comprising: a 4H-SiC bulk single-crystal substrate; and an epitaxial first single-crystal 4H-SiC layer on the 4H-SiC bulk single-crystal substrate and having recesses, wherein the epitaxial first single-crystal 4H-SiC layer has a thickness no smaller than 0.3 μm and no larger than 10 μm, the recesses have respective inner surfaces that consist of smooth curved surfaces, the recesses have diameters no smaller than 2 μm and no larger than 20 μm, the recesses have depths no smaller than 0.01 μm and no larger than 0.1 μm, density of the recesses in the epitaxial first single-crystal 4H-SiC layer is at least 10/cm², and the epitaxial first single-crystal 4H-SiC layer has a defect density no larger than 2/cm².
 9. The single-crystal 4H-SiC substrate according to claim 8, comprising an epitaxial second single-crystal 4H-SiC layer on the epitaxial first 4H-SiC single-crystal layer, burying and completely filling the recesses, without any gaps between the smooth curved surfaces of the recesses and the epitaxial second single-crystal 4H-SiC layer. 